Thermistors
Thermistors
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A thermistor is a thermally sensitive semiconductor
whose resistance varies with temperature
Semiconductor materials: Typically oxides of
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Nickel
Copper
Uranium
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Typically have a large negative temperature
coefficient (NTC)
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Resistance function is non-linear
Static sensitivity is much larger than for RTDs
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Image source: http://image.made-in-china.com/2f1j00tvaEnRwKaGpC/Power-NTC-Thermistors-MF72-.jpg
Manganese
Cobalt
Iron
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Resistance decreases with increasing temperature
Dr. Christopher M. Godfrey
University of North Carolina at Asheville
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How are thermistors made?
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Cut the wires to shape
A dollop of slurry made of mixed metal oxides
and a suitable binder is dropped onto
platinum alloy wires
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Dollop of
thermistor material
The wires are cut to shape depending on the
use of the device
Opposite Cut
Double Adjacent Cut
Uncut strand of beads
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Surface tension forms an ellipsoidal shape
The metals are heated to form an electrical
contact with the wires
Adjacent Cut
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Thermistor Coatings
Glass Probe
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Thermistor Transfer Equation
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The thermistor is usually coated with glass
and/or another protective covering to seal the
thermistor and provide some strain relief
There are several possible transfer equations
for different thermistors
In all cases, the resistance of a thermistor is
a function of the absolute temperature
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Steinhart-Hart Equation
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Studies have shown that a good relationship
for a thermistor exists between ln(RT) and a
polynomial of 1/T
ln RT   b0 
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Steinhart-Hart Equation
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a a 
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RT  exp a0  1  33 
T T 
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b1 b2
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 ...
T T2
RT = Resistance at T (in K)
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Empirical tests have shown that only three
terms are required for accurate
measurements:
This is one version of the Steinhart-Hart
equation
Requires many calibration coefficients
Very nonlinear
Nonlinearities can be corrected with analog
conditioning circuits such as bridge circuits or
by using microprocessors
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Thermistor Transfer Equation
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Thermistor Linearizing Circuit
a a 
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RT  exp a0  1  33 
T T 
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Static sensitivity is not constant, but
decreases with increasing temperature
Amplification would result in HUGE values for
low temperatures
An almost linear circuit with two thermistors
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Thermistor Linearizing Circuit
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Thermistor Advantages
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High resistance (1 k to 100 k)
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Highly non-linear RT vs. T relationships
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Mostly negative temperature coefficients
(NTC) from metal oxides, but positive
temperature coefficient (PTC) models are
available from barium and strontium titanate
mixtures
Can be linearized
Small physical size
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The voltage output (left) and residual nonlinearity (right) from the
linearizing circuit with properties given in your textbook (p. 75)
Eliminates problems with resistance in lead
wires
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Fast response time
Not as small as thermocouples
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Thermistor Advantages
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Easy to manufacture in bulk
Wide temperature range
Very high sensitivity and resolution
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Thermistor Disadvantages
Lower cost than RTDs
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Up to 1000 times more sensitive than
RTDs
Can withstand shock and vibration
Accurate
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Image source: http://www.advindsys.com/Products/Thermistors.htm
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The nonlinear response requires
extra circuitry
Limited range of linear response
with this additional circuitry
Narrow “linear” operating range for
a single sensor
The glass can break if mishandled
Requires an excitation current
High resistance leads to self-heating
errors (more so than RTDs)
Less stable than RTDs
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Comparison of Resistance Thermometers
SS 
Comparison of Resistance Thermometers
dRT
dT
Resistance Temperature Detector
Source: http://www.designinfo.com/cornerstone/ref/negtemp.html
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Resistance Thermometers: A Summary
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The electrical resistance of various materials changes in a
reproducible way when subjected to temperature variations.
An RTD (resistance temperature detector) is a precision
temperature-sensing device that utilizes metal conductors
(typically a fine platinum wire winding or a thin metallic
layer applied to a substrate) and has a positive coefficient
of resistance. That is, as T increases, resistance increases
almost linearly. These are called positive temperature
coefficient devices (PTC).
Thermistors are made from semiconductor materials that
have a large negative coefficient of resistance. That is, as
T increases, resistance decreases. These are called
negative temperature coefficient (NTC) thermistors.
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Other types of temperature sensors:
Capacitive Sensors
C
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K 0 A
d
Once we fix A and d, the only variable that can
change is the dielectric constant, K, which
changes with temperature
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Other types of temperature sensors:
Other types of temperature sensors:
Integrated Circuit (IC) Thermometer
Infrared Camera
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Used in automotive applications and cheap
weather sensors
Sensor and signal conditioning circuits all
located on a single integrated circuit
Accuracy is ~0.5°C
Low cost
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Other types of temperature sensors:
Sensor Choice
Infrared Thermocouple
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Receives heat energy from object and
converts to electrical potential
Unpowered
Low cost
Non-invasive
Fast response time
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How do we decide which type of sensor to use?
Usually depends on your application
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Cost
Size
Additional circuitry
Accuracy
Response time
We can also make a decision by determining which
sensor requires the least amount of gain
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Figure of Merit
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Assuming all other things are equal, look at
the gain:
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Figure of merit = log10 G 1
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We want the smallest gain possible
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High gain amplifier amplifies noise and the signal and is
more expensive
In this example, the thermistor is the best bet, but this may not
be the case for your application!
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